A fiber-reinforced resin material is used as a light, strong material. When a fiber-reinforced resin material includes a compound of a fiber-reinforced base and a matrix of, for example, resin, the dynamic properties (mechanical properties) are improved as compared to the matrix itself. The fiber-reinforced base of a fiber-reinforced resin material includes, for example, a fiber wound body manufactured through filament winding (FW) or braiding (braid). The fiber wound body is manufactured by winding a number of reinforced-fiber yarns about a winding axis to have a tubular or solid structure.
A fiber-reinforced resin material is used as, for example, a load energy absorbing material. When the load energy absorbing material is used as, for example, a crash box arranged between a bumper and a frame of a vehicle body, the crash box generally needs to have a required mechanical strength to hold its position. Additionally, when an impact load exceeding the designed value is applied, the crash box needs to deform and collapse while absorbing the impact load.
The strength of the crash box is lowest in the distal portion, which is a portion that first receives an impact load, and gradually increases toward the basal portion. In a crash box of a fiber-reinforced resin material, the cross section of a reinforced fiber yarn may be increased toward the basal portion of the fiber-reinforced base to gradually vary the strength. For example, patent document 1 discloses a tubular fiber body having a transversal cross section, the shape of which changes along its length. The strength is varied by using a thick fiber yarn in a portion having a large diameter and a thin fiber yarn in a portion having a small diameter.